An ion mobility spectrometer (IMS) detects chemical species in the air by ionizing them and then applying an electric field so that speed of drift caused by the electric field can be measured. Commercial IMS devices are used, for example, to detect explosive and drug residues in security applications.
A conventional ion mobility spectrometer is depicted in FIG. 1(a). As seen in FIG. 1(a), a plasma chromatograph (IMS) chamber 150 includes an envelope 152 of electrodes containing a pair of spaced electrodes 154 and 156. A sample gas may be provided through the inlet 158 and passes through the chamber to the outlet 160. An ionizer 161 is provided adjacent the electrode 154, such that the sample gas passes the ionizer. An electric drift field is established between the electrodes 154 and 156, and a non-reactive drift gas is provided via the drift gas inlet 162. The drift gas fills the region of the chamber between a pair of shutter grids 164 and 166 typically formed as grids of wires in which each alternating wire is held at equal and opposite potentials. The first shutter grid 164 has a mixed ion species population, represented by the letters A, B, and C in FIG. 1(a). The various ion species become segregated in the drift region, and collected at the electrode 156 from which the various ion species may be determined. A conventional IMS device as depicted in FIG. 1(a) is exemplified by the device disclosed in Wernlund, et al., U.S. Pat. No. 3,812,355 issued on May 21, 1974.
In such devices, an air sample is ionized (usually by radioactivity or an electric discharge), and ions are then accelerated towards a detector plate by an electric field applied parallel to the sample gas flow. The current at the detector plate is measured as a function of time. Ions with high mobility (that is high speed when pushed by an electric field) arrive first, while low mobility ions arrive later. Ionic mobility varies non-linearly in high electric fields; therefore, methods have been developed to improve an IMS' ability to identify different chemical species by using a range of different applied electric field strengths.
Design of this type of IMS device is difficult and involves various compromises. For example, the gas typically must be confined during ionization. Otherwise, the gas spreads in the drift direction and accurate measurements of mobility cannot be made. This makes high concentrations of ions difficult to manage and limits the sensitivity of the instrument. The transfer of ions from the ionization chamber into the drift tube is difficult to control. Miniaturizing this design would improve ion losses and create more uniform electric fields, but it would also reduce the resolution as the separation of ions is proportional to the time they spend in the drift tube.
To overcome the problems of miniaturization, an alternative design using an electric field applied transverse to the gas flow has been developed. This type of device is exemplified by the device depicted in FIG. 1(b). As depicted in FIG. 1(b), a gas possibly having an ionized species may enter a measurement region 108 of a flow channel 102 having a channel wall 104. An electric field indicated by the arrow 110 is generated by a source 106. The source 106 may include a plurality of counter electrodes 112 having a voltage applied by a power supply 114. A sensor electrode 118 may be formed of a group 122 of sensor elements 120. Such a device is comparable to the devices disclosed in Murphy et al., U.S. Pat. No. 6,630,663 issued on Oct. 7, 2003, Sacristan, U.S. Pat. No. 5,455,417 issued on Oct. 3, 1995, and Megerle et al., U.S. Pat. No. 5,965,882, issued on Oct. 12, 1999. In such devices, rather than accelerate the ions towards the detector at the end of the drift region, the ions are directed onto the sensor elements of the sensor electrode by a combination of air flow and electric field driven motion. These designs still use a separate ionization chamber and drift region.
If the ions enter the drift region in a zone which is short in the direction parallel to the field, then the mobility of ions can be measured precisely. Such a system is depicted in FIG. 2. As depicted in FIG. 2, an IMS 212 includes an ionizer 204 and a linear electrometer array 210. A gas sample 202 flows through the ionizer 204, which injects the ionized gas sample in the chamber of the IMS 212. A laminar, non-ionized gas flow 206, also referred to in the art as a sheath air, is injected into the IMS which acts as a carrier gas. The laminar gas flow 206 causes larger ions to move faster than smaller ions, and a generated electric field 208 directs ions towards the array 210. The combined effects of the electric field 208 and laminar gas flow 206 causes different ion species to be directed to the array at different points, which permits differentiation of the ion species. A device of this type is exemplified by the device disclosed in Wexler, US 2006/0054804 published on May 16, 2006.
In this manner, precise measurements of the mobility of ions can be achieved using a sheath (or carrier) gas (See also Zhang et al., Int. J. Mass Spec. 258 (2006) at pp. 13-20; Zimmermann et al., Act. B 125 (2007) at pp. 428-434.) In such a configuration, however, the flow rates of the sample and sheath air streams must be balanced. A significant disadvantage of this method is that only a small proportion of the air entering the drift region is ionized, reducing the total detection current and the signal to noise ratio.
Other methods for improving the resolution in a miniaturized device have also been developed. In Field Asymmetric Ion Mobility Spectrometry (FAIMS), detector electrode(s) are placed at the end of the drift tube, and a high frequency alternating electric field is applied within the drift tube. The applied electric field will divert most ions into the channel walls, and only ions of a specific mobility will not be diverted by this field and will reach the detector electrode(s). Such a device is depicted in FIG. 1(c). The FAIMS ion separator of FIG. 1(c) includes an analyzer region 144 defined by two parallel electric plates 138 and 140. A voltage source 136 provides an asymmetric waveform to generate an electric field between the plates. Accordingly, when an ion 132 enters the analyzer region 144 via a gas stream 134, the ion will travel in an exemplary ion pathway 142. As stated above, the ions will tend to be diverted into the walls of the analyzer region and detected. While such systems can be miniaturized and included in an array, only a single ion mobility can be detected at a given time, and the technique is more sensitive to environmental influences. This type of device is exemplified by the devices disclosed in Guevremont et al., U.S. Pat. No. 6,774,360 issued on Aug. 10, 2004, and Zimmermann et al., U.S. Pat. No. 7,244,931 issued on Jul. 17, 2007.